Because the insurance company, if they are changing the rules after-the-fact, are accomplices to the deception
Sorry but the vaccine is experimental, no matter what the Biden of Fauci liars say. Theres no insurance company stupid enough to cover that. If companies are forcing you to get it, then its they that assume the liability. This is their edict. Now if you die of COVID without a shot, then yeah insurance companies should pay up. You take an experimental shot that enhances your ability to get COVID over and over and over again, cancel the policy! This is a racket from big business and big pharma. Sue them.
An essential part of the racket is that the Government has granted big pharma a specific "lawful" exemption from all liabilities for this experimental procedure - and then has "mandated" Federal employees to accept the procedure.
Government has also encouraged big corporations of every sort to "mandate" the experimental procedure as a condition of employment. And those corporations have enthusiastically cooperated.
The Courts have offered weak opposition, and in the case of healthcare workers, have acquiesced to the Executive orders. Basically, every company that accepts Federal contracts will be forced to eventually comply.
This is a mockery of "law" and lawsuits will not settle the conflict. Other means will be used.
As the "All-cause" mortality rates continue to climb, the insurance companies may attempt to protect themselves by not paying claims. That will make them very inviting targets.
Deaths of a large percent of the population tend to make the surviving relatives and friends very angry. Government has every incentive to deflect the wrath of the population towards groups other than Government.
In a Rule-of-Law", the insurance companies might be considered innocent of wrongdoing, should they deny claims, and raise rates. The mRNA treatment is, as you point out, an experimental procedure with considerable, and unknown risks. Perhaps voiding a life-insurance policy might be considered legitimate, even if the patient was coerced into accepting the treatment. I don't personally accept that premise.
But we no longer have a "Rule-of-Law" to settle these conflicts.